Use of high DC-link voltages of power converters can reduce the lifetime of the IGBT transistors due to the impact of cosmic rays. Accordingly, it is desired to keep the DC-link voltage as low as possible. However, in certain situations such as situations where the grid voltage increase above nominal grid voltage, the output voltage amplitude of the power generated by the grid side converter may need to be increased, e.g. in order to prevent a high current flowing from the grid into the grid side converter. There are other situations which also require an increase of the voltage amplitude at the grid side power converter.
EP2655871 A1 discloses a method of operating a wind turbine. The wind turbine comprises a power generator, a generator side converter connected to the power generator, a line side converter connected to a power grid through power components, and a DC-link connected between the generator side converter and the line side converter is provided. The method comprises monitoring the grid voltages on the power grid for overvoltage events and if an overvoltage event is detected, the line side converter is operated in an over-modulation range for at least a part of the duration of the overvoltage event.
Thus, EP2655871 A1 provides a solution to the above-mentioned problem by using over-modulation to increase the output voltage in case of overvoltage situations. However, over-modulation causes grid pollution due to generation of harmonics which are injected into the grid.
Accordingly, there is still a need for improving wind turbines in cases where an increase of the generated output voltage is desired.